1. Field of the Invention
The present invention relates to a microphone unit provided with a function of converting input sound to an electrical signal for output. The present invention also relates to a speech input device provided with such a microphone unit.
2. Description of Related Art
During a telephone conversation, or during speech recognition, speech recording, or the like, it is preferable to pick up only intended speech (the voice of a speaker). However, in environments in which speech input devices are used, sounds other than intended speech, such as background noise, may be present as well. For this reason, speech input devices that have a function of eliminating noise have been developed, making it possible to properly extract intended speech, even in cases of use in environments in which noise is present.
In recent years, there have been dramatic enhancements in the functionality of mobile devices such as mobile terminals, smartphones, and the like, in which there have aggressively started to be installed not only normal speech conversation, but also functions such as hands-free conversation, videophone functionality, speech recognition, and the like. Techniques by which devices having such functions may be made smaller and thinner have assumed increasing importance.
Omnidirectional microphones, which have a circular directionality pattern, are known as microphones that are adapted to pick up sound uniformly from all directions. Additionally, unidirectional microphones, which have a directionality pattern of a cardioid type, are known as microphones that are adapted to pick up sound from a particular direction. Moreover, bidirectional microphones, which have a figure “8” directionality pattern, are known as microphones that are adapted to minimize distant sounds, and to pick up nearby sounds only. These microphones are used selectively according to particular applications and purposes for use.
An omnidirectional microphone has a single sound hole, and is designed so that sound pressure inputted through the sound hole is transmitted to the front surface of a diaphragm of the microphone and the back surface of the diaphragm faces an enclosed region imparted with a baseline pressure.
A bidirectional microphone has two sound holes, and is designed so that sound pressure inputted through one of the sound holes is transmitted to the front surface of the diaphragm of the microphone, while sound pressure inputted through the other sound hole is transmitted to the back surface of the diaphragm, to thereby detect a pressure differential between the sound pressure inputted through the two sound holes (see, for example, Japanese Laid-open Patent Application No. 2003-508998).
A unidirectional microphone has two sound holes, and is designed so that sound pressure inputted through one of the sound holes is transmitted to the front surface of the diaphragm of the microphone, while sound pressure inputted through the other sound hole is transmitted to the back surface of the diaphragm through a delay member that imparts an acoustic delay, to detect a pressure differential between the sound pressure inputted through the two sound holes (see, for example, Japanese Laid-open Patent Application No. 2008-92183).
An example of a unidirectional microphone unit 101 is shown in FIG. 33. A substrate opening 106 that passes from the front surface to the back surface of a substrate is formed in a substrate part 102, and a diaphragm 103 is installed thereon in such a way as to block the substrate opening 106.
A cover 104 is installed over the substrate part 102, so as to cover the diaphragm 103, and the outer edge of the cover 104 is hermetically joined to the outer edge of the substrate part 102, forming an internal space that includes the diaphragm 103. The cover 104 is furnished with a sound hole 107, and sound pressure inputted from the outside is transmitted from the sound hole 107 to the front surface of the diaphragm 103, via the internal space.
An acoustic delay member 105 is disposed in such a way as to block the substrate opening 106 from the back side, and the unidirectional microphone is configured in such a way that sound pressure inputted from the outside passes through the acoustic delay member 105, and is transmitted to the back surface of the diaphragm 103 via the substrate opening 106. Felt material or the like is widely used as the acoustic delay member 105. Instead of being disposed to the back side of the substrate opening 106, the acoustic delay member 105 can be disposed in such a way as to block the sound hole 107 of the cover 104, as shown in FIG. 34.
Another method for configuring a unidirectional microphone is a configuration as shown in FIG. 35, in which two omnidirectional microphones are respectively mounted on the upper surface and the lower surface of a substrate part 102, the sound holes of the two microphones (a first sound hole 113 and a second sound hole 114) are disposed in such a way as to face up and down in opposite directions, and arithmetic operations are performed on the output signals of the respective microphones (see, for example, Japanese Laid-open Patent Application No. 2008-92183).
In recent years, the need to make mobile terminals and other such mobile devices even thinner has become increasingly intense. To meet this need, thinner omnidirectional microphones employing microelectromechanical systems (MEMS) have been developed, and microphones 1 mm or less in thickness have become commercially viable.
Meanwhile, in the case of unidirectional microphones such as shown in FIG. 33 and FIG. 34, it is necessary for the thickness of the unidirectional microphone to be equal to the thickness of the substrate part 102 and the cover part 104, plus the thickness of the acoustic delay member. A resultant problem is that, due to the additional thickness, reducing thickness becomes difficult.
According to another method, a unidirectional microphone is configured, as shown in FIG. 35, by respectively mounting two omnidirectional microphones on the top and bottom surfaces of a mounting substrate, and performing arithmetic operations on the output signals of the respective microphones. However, problems are presented in that, because the thickness of the resulting microphone is approximately doubled, reducing thickness becomes difficult.